Measurement of the concentration and level of filling of vessels which hold saline solutions can be carried out by what is referred to as the so-called bubbling-in method. The measuring gas is introduced into the liquid through air-bubbling pipes. The density or filling level can be ascertained by the pressure differential as disclosed in the text "Chemie der Nuklearen Entsorgung", Part II, Thiemig-Taschenb0cher, Vol. 66, pages 203 to 213 (1978) and published by Verlag Karl Thiemig, Munich.
If this pressure differential method is used for saline liquids or for liquids which tend to crystallize out, for example, in an evaporator, then the measuring air extracts moisture from the solution so that crystallization can occur in the supply pipe, that is, in the region of the air outlet opening of the bubbling-in pipe. In this unfavorable situation, the pressure differential increases with increasing operating time and simulates operating conditions which deviate from the actual. It is possible for the measuring line to finally become completely blocked. In addition to the incorrectly measured value, frequent maintenance is regarded as a disadvantage.
Saline solutions can also be propelled by air-lift means as disclosed on page 210 of the above-mentioned text "Chemie der Nuklearen Entsorgung". Here, too, there is the risk of clogging.
The problem of clogging has been countered by using larger pipe diameters for the pressure measuring means. However, they provide inaccurate results as noted in the text "Engineering for Nuclear Fuel Reprocessing", by Justin T. Long, page 735, American Nuclear Society 1978.